This invention pertains generally to methods of operating a nuclear reactor and more particularly to such methods that accommodate adverse operating conditions without tripping the reactor.
Generally, nuclear reactors contain a reactive region commonly referred to as the core in which sustained fission reactions occur to generate heat. The core includes a plurality of elongated fuel rods comprising fissile material, positioned in assemblies and arranged in a prescribed geometry governed by the physics of the nuclear reaction. Neutrons bombarding the fissile material promote the fissionable reaction which in turn releases additional neutrons to maintain a sustained process. The heat generated in the core is carried away by a cooling medium, which circulates among the fuel assemblies and is conveyed to heat exchangers which in turn produce steam to drive a turbine for the production of electricity.
Commonly, in pressurized water reactors a neutron absorbing element is included within the cooling medium (which also functons as a moderator) in controlled variable concentrations to modify the reactivity and thus the heat generated within the core, when required. In addition, control rods are interspersed among the fuel assemblies, longitudinally movable axially within the core, to control the core's reactivity and thus its power output. There are three types of control rods that are employed for various purposes. Full length rods, which extend in length to at least the axial height of the core when fully inserted, are normally employed for reactivity control. Part length control rods, which have an axial length substantially less than the height of the core, are normally used for axial power distribution control. In addition, reaction shutdown control rods are provided for ceasing the sustained fissionable reaction within the core and shutting down the reactor.
In one type of reactor currently in commercial use movement of the part length rods and full length control rods is controlled by magnetic jack mechanisms similar to the jack mechanisms described in U.S. Pat. No. 3,158,766, by E. Frisch which incrementally move the control rods into and out of the core to obtain the degree of reactivity control desired. Generally in this type of arrangement, commonly referred to as rod cluster control, the control rods are inserted and withdrawn from the core in prearranged groups in a preselected order.
In a second type of nuclear reactor the control rod drive mechanisms employ electrically controlled, hydraulically operated devices which individually move control rods between only two rest positions; either full in or full out. Each control rod has at least one neutron absorber element associated with it, approximately the same size as the fuel rods. Each control rod is connected to a hydraulic mechanism and travels in guide thimbles provided within each fuel assembly. In the inserted position, the respective absorber elements fit in corresponding fuel assembly thimbles and in the withdrawn position they are completely removed and located in guide tubes in the reactor head. This type of arrangement is similar to the first type of reactor control arrangement described, except that the control rods employing hydraulic drive do not assume any intermediate positions other than being either fully inserted or fully withdrawn. Each hydraulic control mechanism is a completely independent system and controls the movement of one drive rod which is connected to the absorber rods associated with one control rod. Eight of these independent mechanisms are generally located in a single control assembly. However, each mechanism must have its own hydraulic valve coil and position indicator to operate completely independent of the other seven. For a better understanding of the operation of such a control rod system, reference can be had to U.S. Pat. No. 3,519,535 filed Jan. 24, 1968 by Robert J. French et al entitled "Nuclear Reactor" and U.S. Pat. No. 3,742,409 entitled "Magnetic Position Indicator" by Dean C. Santis et al.
Presently, the probability of reactor trip (the complete cessation of the core reactions) is extremely high in nuclear facilities employing present reactor control systems which manipulate the various sets of control rods in response to the loss of a major reactor component such as a large pump. In fact, in many reactor systems in use to date, the loss of a main reactor coolant pump during operation of the reactor above a preset power level will directly trip the reactor and shut down the plant. However, practically this type of operation is an expensive over-reaction to the malfunction that would not be necessary if the reactor power could be cut back rapidly to a safe level compatible with operation with the malfunctioning component. While desirable, this latter type of operation has been avoided since it would require a complete change in control pholosophy from that presently employed. To date, most reactors derive their power control signals from the load output. A controlled change in reactivity upon the occurrence of a malfunction would require control of the load output as a function of the reactor output to maintain an equilibrium condition.
Accordingly, a reactor control system is desired that is responsive to the occurrence of selective malfunctions of operating components to rapidly reduce the reactor power output and correspondingly control the load output, changing control from a reactor following mode of operation to a load following reactor mode.